surveillance. >> afederallawenforcementofficial agrees and says tamerlan was not on a terror watch list or any no-fly list because the u.s. never deemed him a threat. so there were no alarm bells when tsarnaev came back to the u.s. six months later. >> but the time he returned, all investigations, the matter had been closed. >> but even sew, it's not clear if the department of homeland security was charged with monitoring travel even know that ztsarnaezatsarnaev was on his r. >> we are trying to make sure that all of that information that was available was shared. if it wasn't, there may be somebody who dropped the ball. >> a u.s. official said even when there's a hit in the system it doesn't prompt anyone in law enforcement to take action. it's just monitoring for suspicious travel. joe johns, cnn, washington. zbr well, we're getting more breaking news now. a u.s. official telling cnn's jessica yellin about the ongoing investigation that there is no hard evidence of accomplices. there is no known evidence to extremists. they're still not certain what radicalized them. additionally inve